Nonenzymatic glycation of albumin, the major serum protein, is a structural modification that can affect the biologic properties of albumin and increase the susceptibility of diabetic subjects to kidney and other complications of diabetes.
Nonenzymatic glycation is a condensation reaction between carbohydrate and free amino groups at the amino-terminus of proteins or at the epsilon amino groups of lysine residues of proteins. The reaction is initiated with attachment of the aldehyde function of acyclic glucose to a protein amino group via nucleophilic addition, forming an aldimine, also known as a Schiff base. This intermediate product subsequently undergoes an Amadori rearrangement to form a 1-amino-1-deoxyfructose derivative in stable ketoamine linkage, which in turn can cyclize to a ring structure (Cohen M. P., Diabetes and Protein Glycosylation, Springer Verlag, 1986). This bimolecular condensation of free saccharide with protein constitutes a mechanism by which proteins are subject to post-ribosomal modification without the influence of enzymatic activities. In diabetic subjects, hyperglycemia promotes increased nonenzymatic glycation of circulating and tissue proteins, which has allowed the assessment of integrated glycemic control through determination of circulating glycated protein levels. The increased glycation of proteins may also provide insight into the pathogenetic mechanisms responsible for the chronic complications associated with diabetes.
Experimental studies have suggested several ways in which glycation of albumin could theoretically contribute to the development of kidney and other complications associated with diabetes. These include the induction of conformational changes and alterations in the ligand binding properties of albumin (Shaklai, et al., J. Biol. Chem. 259:3812, 1984), and the induction of enhanced transendothelial passage of albumin (Williams, et al., Proc. Natl. Acad. Sci. USA, 78:2393, 1981; Williams & Solenski, Microvasc. Res. 28:311, 1984; Williams & Siegal, Kid. Int. 28:146, 1985). Such experimental studies have used albumin glycated in vitro to explore effects on its biological properties. Deposition of glycated albumin in extracellular matrices (Miller, et al., Diabetes, 25:701, 1976; Michael & Brown, Diabetes 30:843, 1981; McVerry, et al., Lancet 2:738, 1980), and the formation of advanced glycation end products (Cohen, M P, supra, 1986) also have been suggested to be involved in the development of kidney disease and other complications.
Electron microscopic work indicates that nonenzymatically glycated albumin is taken up more avidly than native albumin by endothelial cells. Endothelial cell uptake involves micropinocytic vesicles which participate in the bidirectional transport of proteins across the capillary wall. Additionally, glycation promotes the preferential transport of albumin across the glomerular filtration barrier. Carbohydrate-free protein perfused into kidneys accumulates within the lamina rara interna and is restricted from transglomerular passage, whereas glycated protein penetrates the lamina densa and the lamina rara externa and accumulates in epithelial pinocytic vesicles and multivesicular bodies. The increased capillary permeability in diabetes has been ascribed, in part, to the increase in transendothelial transport that glycation of albumin confers. This increased transport and facilitated penetration of the basement membrane may cause deposition of glycated albumin in the subendothelial face of the filtration barrier and in the appositioned cellular elements. Other studies have suggested that glycated albumin may accumulate in and/or injure tissues by binding to proteins in the cell or matrix (Brownlee, et al., Diabetes 34:938, 1985; J. Exp. Med. 158: 1739, 1983) and by forming immune complexes (Vaughn, et al., Clin. Immunol. and Immunopath. 52:414, 1989) or advanced glycation end products (Cohen, M P, supra, 1986). Also, the injection of glycated plasma proteins has been reported to produce glomerular basement membrane thickening in nondiabetic mice (McVerry, et al., Lancet 2:738, 1980) and hyperfiltration in nondiabetic rats (Sabbatini, et al., Kid. Int. 42:875, 1992).
Histochemical studies have suggested that endogenous albumin can bind to kidney basement membrane (Miller, et al., Diabetes 25:701, 1976) and that there is increased deposition of albumin in the basement membranes of patients with diabetic nephropathy (Michael, et al., Diabetes 30:843, 1981). However, such studies did not distinguish between glycated and unglycated albumin. Even if glycated albumin does accumulate in kidneys or other tissues in diabetic subjects, it is not known whether the deposited glycated albumin retains its fructosyllysine chemical configuration. In fact, prior art has suggested that the fructosyllysine group of glycated albumin that is deposited in tissues is obscured through the formation of advanced glycation end products and cross-links (Brownlee, et al., Science 232:1629, 1989).
Thus, there is a need in the art to determine whether glycated albumin accumulates in the kidneys and other tissues of diabetic subjects and to determine whether detection of accumulated glycated albumin can be used to determine the course of treatment. There is also a need in the art for a means to prevent the deleterious cell and tissue effects of glycated albumin that contribute to the development of complications of diabetes.